GSUB selects glyph forms
The Glyph Substitution table can replace sequences with ligatures, contextual forms, reph forms, or below-base consonants. These rules refer to glyph IDs, not visual similarity.
GPOS positions the result
The Glyph Positioning table adjusts marks, bases, cursive attachments, and pair spacing after substitution. Preserving anchors and lookup references is essential when outlines change.
GDEF supplies glyph roles
The Glyph Definition table classifies bases, ligatures, marks, and components. Shaping lookups use those classes to decide where rules apply, so a safe outline editor leaves this structure intact.